Allow env var prefix overrides

[devth]

No description provided.

[codecov]

Codecov Report

Merging #126 into master will increase coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   42.12%   42.13%   +0.01%     
==========================================
  Files          91       91              
  Lines        3877     3880       +3     
  Branches      168      168              
==========================================
+ Hits         1633     1635       +2     
- Misses       2076     2077       +1     
  Partials      168      168              

Impacted Files	Coverage Δ
src/yetibot/core/config.clj	84.61% <75.00%> (-2.35%)	⬇️

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